Recently, a light-emitting type flat panel display, such as an organic electroluminescence (EL) display device, has been widely noticed. Such an organic EL display device, for example, consists of a large number of pixels, each of which is provided with a fine organic EL display element.
The organic EL display device has the following advantages:
(1) Since the EL display element is a light-emitting type, a bright and sharp display with a wide viewing angle can be achieved.
(2) No rear light source is required so that the display device is thin in thickness and light in weight.
(3) Since the display device can be driven by a direct current, it is not subjected to noises.
(4) A response speed of the EL display elements is fast in the order of μsec so that smooth motion pictures can be reproduced.
(5) Since the EL display element is a solid-state light-emitting device, it is possible to widen a service temperature range.
For the advantages as described above, the EL display device has been aggressively developed.
On the other hand, an active matrix display device provided with poly-crystalline-silicon thin-film transistors has been already put into a practical use. A poly-crystalline-silicon semiconductor layer for thin-film transistors is made of a poly-crystalline-silicon film into which an amorphous-silicon film formed on a substrate is re-crystallized and to which a prescribed patterning process is then applied.
There are prior art laser-annealing methods in which pulsed-laser beams are irradiated to melt and re-crystallize an amorphous-silicon film into a poly-crystalline-silicon film. According to Japanese Unexamined Patent Publication No. Hei 11-121751, for instance, laser beam scanning directions are consistent with the width direction of a channel region of the semiconductor layer to prevent discontinuities at the semiconductor layer of pixel-switching transistors (disconnections between channel regions and source/drain regions of thin-film transistors) caused in a liquid crystal display device.
One of the prior art laser annealing methods teaches that a longitudinal direction of gate electrodes of thin-film transistors is perpendicular to that of crystalline grains to obtain uniform-property pixel-switches used in a liquid crystal display device (as described in Japanese Unexamined Patent Publication No. 2000-243969, for instance). Another prior art laser annealing method describes that a longitudinal direction of gate electrodes of thin-film transistors is at an angle of 45° with respect to that of crystalline grains (as pointed out in Japanese Unexamined Patent Publication No. 2000-243968, for instance).
Where these prior art methods are applied to an active matrix type liquid crystal display device, even though there is more or less dispersion in grain diameters of silicon crystals constituting semiconductor layers for pixel switches, improvements in their switching properties can be expected. In the case of an active matrix type light-emitting display device, however, the dispersion in grain diameters of the silicon crystals may cause the display device uneven brightness.
Such an active matrix type light-emitting display device is provided with pixel switching elements and driving elements to control a driving current of light-emitting elements in response to video signals supplied through the pixel switching elements. Where both pixel switching and driving elements are composed of thin-film transistors, the semiconductor layers of which are made of poly-crystalline-silicon films, the dispersion in crystallinity of silicon of the driving elements, particularly, results in different carrier mobility in their channel regions. Thus, even though the light-emitting elements are driven by means of substantially the same-level-video signals, dispersion occurs in driving-current-supply capability of the driving elements. In the case of uniform image display, high and low bright pixels are mixed up so that the image quality may become deteriorated.
The dispersion in crystallinity of silicon is most likely to be caused by the following reasons. Although a poly-crystalline-silicon film is formed by irradiating laser beams to melt an amorphous-silicon film and then by crystallizing the same, the crystallinity of such a poly-crystalline-silicon is finally determined by the shot of irradiated laser beams. In other words, the laser beam power is supposedly set to be uniform but some shots of the irradiated laser beams are actually higher or lower in power than the remaining ones of the irradiated laser beams. Such irregular power shots of the irradiated laser beams may make the crystallinity of silicon different.
Since the driving elements connected in series with the light-emitting devices are, however, disposed in the same location as in the row pixels perpendicular to a scanning direction of the laser beams, each of these driving elements has a poly-crystalline-silicon film finally formed by the same shot of laser beam. Thus, the driving elements disposed in the direction perpendicular to the scanning direction of laser beams have substantially the same mobility but those disposed in the scanning direction of laser beams have some dispersion in mobility.
Since high mobility driving elements are highly capable of supplying currents to the light-emitting diodes, the brightness of the light-emitting diodes driven by the high mobility driving elements becomes high, and vice versa. Thus, the mobility of driving elements depends on the shots used for making the poly-crystalline-silicon films of the driving elements and the brightness of the light emitting diodes has dispersion in accordance with the shots so that line-like uneven brightness appears to extend in the direction perpendicular to the scanning direction of the laser beams.
As described in detail above, some thin-film transistors provided with semiconductor layers made from poly-crystalline-silicon films to drive light-emitting elements have certain differences in crystallinity of silicon due to irregular power shots of laser beams in an annealing process to form the poly-crystalline-silicon films. The differences in crystalline silicon give rise to different magnitudes of driving currents supplied to light-emitting elements under the same level of voltage. Such different magnitudes of driving currents cause uneven brightness among pixels.